Office copiers and printers utilizing the xerographic process of toner development of a latent image on a photoreceptive surface, are becoming more sophisticated with the use of integrated and other type of high technology circuits. In addition, to increase the reliability of the copier or printer and to render as high quality an output document as possible, certain process variables such as toner density and charge distribution on the photoreceptor must be controlled. However, measurements of these factors can be misleading or false due to errors in optics, the optoelectronic chain, and certain environmental effects. Thus, it would be desirable to fabricate control circuitry to allow the measurement of the relative optical density of the developed toner and other parameters, but overcome the deleterious error effects as set forth above.
According to the present invention, an integrated circuit chip is provided with digital and analog circuits thereon for providing a low cost infrared reflectance densitometer for detecting relative toner density on a photoreceptive surface including several stages of calibration including photodiode detection means for monitoring and controlling the light output from a light emitting diode, photodiode means for detecting undesirable scattered background light signals, further photodiode means for detecting the light reflected from said photoreceptive surface as may be affected by toner deposited thereon, an automatic gain control circuit for automatically adjusting the output gain of the reflectance densitometer, and sample and hold circuit means to adjust the circuit for different effects of the aging, leakage current effects or other performance characteristics of the circuit components.